1. Field of the Invention
This invention concerns heat sealed polymeric films and the formation of such seals with such films.
2. Description of Related Art
Polymeric films used for packaging are usually required to have good slip properties both when hot, so that the film slides satisfactorily on metal surfaces of packaging machines, and when cold, so that the packages themselves can slide on each other. Particularly demanding conditions occur in the formation of so-called "drag-seals" where film to be sealed is pulled across the face of a hot heat surface before heat sealing is effected, compared with situations in which the film is required to slide over other hot metal surfaces, for example in form-fill-seal machines.
Increased slip can be provided by the use of larger amounts of slip and/or antiblock agents. However, increasing the amount of such agents often reduces the optical properties of the films, relatively large amounts of such agents often increasing the haze of the films. In the case of increased amounts of inorganic antiblock agents such as silica or zeolites, the result can be scratching of the film by film-to-film contact caused by the abrasive effect of the antiblock agent.
The result of these problems is that a trade off is made between the speed of packaging which can be carried out and the optical properties of the film being used.
One proposal for dealing with this problem has been to use organic antiblock agents. However, hitherto proposed organic antiblock agents often have relatively low softening temperatures and/or melting points, which leads to poor hot slip properties even if their room temperature slip properties and haze are otherwise satisfactory. Furthermore, the temperatures used to extrude the polymers used to produce polymeric films are often sufficiently high that hitherto proposed organic antiblock agents degrade at the extrusion temperatures, leading to the build up of degraded material at the die. This can happen, for example, when using polyamide antiblock agents. Hitherto proposed polyalkyl methacrylate antiblock agents have also been found to reduce in size during extrusion, thereby leading to a reduction in their effectiveness as an antiblock agent.